1. Field of the Invention
This invention relates to a digital compass in which direction of the Earth's magnetic field is determined on the basis of frequency differences which are a function of the orientation of the digital compass with respect to the Earth's magnetic field. More particularly, it relates to such a digital compass in which it is not necessary to convert analog signals to digital signals. It further relates to a novel form of a magnetometer which has general application for determining orientation of weak magnetic fields based on frequency differences.
2. Description of the Prior Art
A variety of digital compasses and magnetometers are known in the art. For example, U.S. Pat. No. 3,396,329, issued Aug. 6, 1968 to Salvi, discloses a magnetometer in which the intensity of weak magnetic fields is a function of frequency difference in sensed signals, but independent of orientation of a vessel in which the magnetometer is installed. U.S. Pat. No. 3,634,946, issued Jan. 18, 1972 to Star, relates to an all digital circuit implementation of a digital compass which operates on the basis of spatial relationships of pulses produced when a sensor is aligned in a reference direction and orthogonal to the Earth's magnetic field. There is no mention in this patent of frequency differences created by orientation, nor does the circuit shown discriminate on the basis of such frequency differences. U.S. Pat. No. 4,305,034, issued Dec. 8, 1981 to Long et al., discloses a magnetometer in which frequency changes are created when a background magnetic field, which can be the Earth's magnetic field, is perturbed by a metal object, but this device cannot provide sign information, i.e., whether the field is parallel or antiparallel to the sensor coil. U.S. Pat. No. 4,340,861, issued July 20, 1982 to Sparks, discloses a magnetometer in which frequency differences are used to determine distribution of magnetic fields produced by permanent magnets, on the basis of amplitude information in the different frequency signals. U.S.S.R. Pat. No. 945,835, issued July 27, 1982 to Bondarevsk et al., discloses that a strong magnetic field will produce frequency differences in an LC circuit.
The following additional issued U.S. patents relate to digital compasses which utilize phase differences, comparison with previous signals at known orientations or counting of sensing marks to determine orientation: No. 3,490,024, issued Jan. 13, 1970 to Sherrill et al.; No. 3,903,610, issued Sept. 9, 1970 to Heaviside et al.; No. 3,952,420, issued Apr. 27, 1976 to Benjamin et al.; No. 4,095,348, issued June 20, 1978 to Kramer; No. 4,179,741, issued Dec. 18, 1979 to Rossani; No. 4,424,631, issued Jan. 10, 1984 to Franks and No. 4,640,016, issued Feb. 3, 1987 to Tanner et al. The following issued U.S. patents relate generally to magnetometers: No. 3,432,751, issued Mar. 11, 1969 to Godby et al.; No. 3,435,337, issued Mar. 25, 1969 to Inouye et al., No. 3,461,387, issued Aug. 12, 1969 to Morris et al., No. 3,768,011, issued Oct. 23, 1973 to Swain and No. 4,641,094, issued Feb. 3, 1987 to Dalton, Jr. The state of the art in magnetometer design is further indicated by Takeuchi et al., "A Resonant-Type Amorphous Ribbon Magnetometer Driven by an Operational Amplifier," IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 1723-1725.
While the art relating to the design of digital compasses and magnetometers is thus a well-developed one, a need remains for development of a simple, reliable, low cost digital compass suitable for consumer use and a simple magnetometer for determining orientation of low intensity magnetic fields.